The present invention relates to interaction of human platelets with blood vessels mediated by von Willebrand factor. More particularly, the invention relates to a method of inhibiting thrombin or ADP-induced binding of von Willebrand factor to human platelets as well as the construction of a synthetic molecules capable of initiating the adhesion reaction linking human platelets to blood vessels.
A number of different mechanisms have been disclosed which lead to the initiation of blood clots (thrombi). One such mechanism is the thrombin or ADP-induced platelet binding of the multivalent molecules, fibrinogen or von Willebrand factor. Thrombin or ADP induce modification of the platelet structure, allowing interaction between the platelets and fibrinogen to form aggregates and binding of von Willebrand factor by the platelets to initiate adhesion or linking to blood vessels. Von Willebrand factor is also involved in the ristocetin-induced aggregation (clumping) of platelets. While the exact mechanisms are not clear, one theory is that thrombin, ADP or ristocetin cause stereochemical changes in the glycoproteins of the platelet cell membrane. The stereochemical changes create receptor sites on the platelet so that platelet binding regions on the fibrinogen or von Willebrand factor molecules can react with the receptor site. Until recently, the mechanisms of binding of human fibrinogen or von Willebrand factor to human platelets were unknown. In 1982, Hawiger, Timmons, Kloczewiak, Strong and Doolittle demonstrated that the primary fibrinogen interaction site with human platelets is located on the gamma chain of fibrinogen. See Proc. Natl. Acad. Sci. USA 79:2068-2071 (1982). In a later paper, Kloczewiak, Timmons and Hawiger demonstrated that the platelet binding region was contained within the carboxyl terminal 27 peptide residues of the gamma chain and that a 15 peptide carboxyl terminal fragment of this molecule could block fibrinogen platelet aggregation. See Biochem. and Biophy. Rsc. Comm. 107:181-187 (1982). In 1982 Fujimoto, Ohara, and Hawiger, and Fujimoto and Hawiger described the interaction between ristocetin, thrombin, or ADP-modified platelets and von Willebrand factor molecules. See J. Clin. Invest. 69: 1212-1222 (1982); Nature (London) 297: 154-156 (1982).
Both fibrinogen and von Willebrand factor are important in the formation of hemostatic platelet plugs and initiation of thrombotic lesions. Blockage caused by these plugs and the damage caused by thrombotic lesions are major factors in heart disease and stroke. Much research has been directed toward developing drugs which will dissolve already formed blood clots but most of these drugs have not been particularly effective. Recently, reports on the use of tissue plasminogen activator, a molecule which modifies circulating plasminogen molecules to form plasmin, an enzyme that dissolves blood clots, have been given much publicity. While enzymatic methods of dissolving clots may help minimize the after effects of heart attacks, a pharmaceutical preparation which will inhibit platelet adhesion prior to occlusion of the blood vessels may prevent the initial blockage responsible for cardiac or cerebral infarction.
Alternatively, molecules promoting platelet adhesion to blood vessels may have a variety of uses. For example, a number of patients, e.g., some bleeders, may be lacking von Willebrand factor due to a genetic deficiency or due to excessive consumption in circulation. A synthetic platelet adhesion promoting molecule can assist in platelet plug formation and attachment of plugs to blood vessels to arrest bleeding and help these patients to lead normal lives.
Accordingly, an object of the invention is to develop a method of inhibiting platelet binding of von Willebrand factor promoted by thrombin, ADP or other stimuli. Another object of the invention is to provide a molecule which promotes platelet interaction with blood vessels. A further object of the invention is to provide a synthetic molecule which inhibits platelet binding of von Willebrand factor with significant circulation time in the blood stream. These and other objects and features of the invention will be apparent from the summary, the drawing and the description.